The present disclosure relates to methods and systems for electrical stimulation of the expiratory muscles to produce cough in human patients or other mammals with spinal cord injuries resulting in the paralysis of their expiratory muscles.
The sequence of cough is divided into four phases: inspiration, compression, expiration and cessation. During inspiration, a variable amount of air is inhaled. The greater the amount of air inhaled, the larger the force that can be developed by the expiratory muscles. As lung volume increases, the expiratory muscles progressively improve in their pre-contractile length. During compression, the glottis closes followed by expiratory muscle contraction. When the expiratory muscles contract against a closed glottis, intrathoracic and intraabdominal pressures become quite high and may exceed 200 cmH2O. When pleural pressure increases, lung volume decreases as the intrathoracic gases are compressed. The glottis usually remains closed for about 200 milliseconds (msec). The opening of the glottis signals the onset of the expiratory phase of cough. This is an active process associated with passive oscillations of tissue and gas. These oscillations cause the noise generally characteristic of cough. Following glottic opening, intrathoracic pressure drops rapidly toward atmospheric levels, whereas the pressure at the alveolar level remains positive and actually continues to rise for a short while. The high intraalveolar pressures simultaneously promote high expiratory flow rates and tend to collapse central airways. Accordingly, the transient peaks of expiratory flow noted during cough represent both the sum of the flow related to the displaced volumes of gas in the central airways which are dynamically collapsed and also the flow of gas from the distal parenchymal units passing through these collapsing airways. These transient peaks play an important role in the movement of foreign material toward the airway opening. The final phase of cough is characterized by the relaxation of the expiratory muscles and the antagonistic activity of the diaphragm and other inspiratory muscles and the return to normal breathing.
Patients with spinal cord injuries involving the T5 level or higher suffer from paralysis of their expiratory muscles, including the lower intercostal and abdominal muscles, which are the major muscles for the development of an effective cough. There are about 250,000 patients with spinal cord injury in the United States, with about 12,000 new injuries occurring each year. Within this group, about 40 to about 50 percent of these patients have cervical spinal cord lesions resulting in tetraplegia. Following spinal cord injury (SCI), respiratory complications account for most of the morbidity and mortality in this patient population.
Despite intensive respiratory management, these patients frequently develop atelectasis (lung collapse), bronchitis and pneumonia. In epidemiological studies of over 5,000 patients sustaining SCI, the leading cause of death was pneumonia. The development of respiratory complications in this patient group is directly related to their inability to cough and clear secretions. The lack of an adequate cough defense system occurs as a consequence of paralysis of virtually all of their expiratory muscles. Although ciliary activity is an important mechanism of mucus clearance, coughing is necessary when the cilia are ineffective in removing secretions or overwhelmed by excessive secretions.
Strokes also account for 5 million deaths a year worldwide; 30% of patients with stroke die within 3 months. Most deaths are caused by stroke-related complications, of which respiratory tract infections are the most common. Absent or weak voluntary cough in stroke patients has been associated with a higher incidence of aspiration and respiratory infections. Restoration of an effective cough and associated reduction in incidence of respiratory infections would substantially decrease death rates after stroke.
Currently employed techniques to manage airway secretions include patient positioning, active suctioning, manual assistance by abdominal compression (assisted cough) and/or use of a mechanical insufflation-exsufflation device.
Patient positioning places the patient in relatively awkward positions for prolonged periods to remove secretions by gravity, and is quite uncomfortable. In active suctioning, a suction catheter is repeatedly introduced into the lower respiratory tract. This method is uncomfortable and can result in tracheal injury, irritation, hemoptysis, and respiratory tract infections. This method also does not facilitate removal of secretions from small airways where they are produced and often results in gas exchange abnormalities. With regard to assisted cough and the mechanical insufflation-exsufflation device, these techniques do not result in uniform distribution of pressure within the intrathoracic cavity and, therefore, have limited effectiveness in many patients. Another major disadvantage of these methods is that they require the presence of trained personnel, and thus are time-consuming, costly, and labor-intensive.
Functional electrical stimulation of the expiratory muscles has the potential to provide a normal and effective cough mechanism. There are three methods by which the expiratory muscles can be activated to produce cough: (1) high frequency magnetic stimulation; (2) surface abdominal muscle stimulation; and (3) lower thoracic spinal cord stimulation (SCS).
Magnetic stimulation is an experimental device which can be applied to the lower back to activate the neural pathways innervating the expiratory muscles. In normal subjects, this technique has been shown to result in the generation of large positive airway pressures and expiratory flow rates. One major advantage of this technique is that it can be applied non-invasively. However, several disadvantages are also present. Initially, airway pressures and airflow rates produced during magnetic stimulation are not significantly different from that which could be generated during maximal spontaneous efforts, and can be quite low. Next, this device requires the application of current applied at high stimulus frequencies (>20 Hz), which generates substantial heat at the stimulating coil and consequently carries the risk of thermal injury. In addition, this device is quite large, requires an external power source, and is not very portable. This is a substantial disadvantage since patients can still be mobile and cough is often required on an emergent basis. Fourth, large amounts of adipose tissue (common in patients with SCI) may preclude successful stimulation in obese patients. Finally, the presence of a trained caregiver is still required to properly apply the device.
Direct stimulation of the abdominal muscles with electrodes positioned over the surface of the abdominal wall has been tried. In several previous animal and human studies, increases in positive airway pressure were marginal in the range of ˜30 cmH2O. In some studies, employing surface electrodes with much larger surface areas, pressure generation was quite substantial in the range of 150 cmH2O. As with magnetic stimulation, this method also has the advantage of being non-invasive. However, the repeated application of electrodes over the skin may lead to thermal injury, irritation, and breakdown. Also, significant adipose tissue may interfere with its successful application in obese patients due to the high electrical resistance of fatty tissue. Repeated application of electrodes to the skin surface may also prove to be quite tedious and cumbersome.
Lower thoracic spinal cord stimulation (SCS) currently involves the use of disc-shaped or paddle-shaped leads, which are essentially a two-dimensional substrate with an electrode on one side. Such leads are typically installed, repositioned, or removed using one or more laminectomies, which is a major invasive surgery in which portions of the vertebral bone are removed and which can take four to six hours and require two to three days of hospital admission. In addition, the leads can be difficult to position accurately and the leads can change position.
Many patients find the current technology, which involves an invasive procedure, acceptable given the potential benefits. However, it would be desirable to provide additional methods that could be less invasive.